There are many applications where reliable imagers located in space can provide important information. Two examples of these applications include weather monitoring and strategic military surveillance. In both of these applications, it is important for high resolution images to be transmitted to the Earth in a very short period of time. For example, tornadoes develop so quickly that a weather image transmission technique, in which images can be transmitted within a few minutes, provides very important information.
It is desirable that any satellite imager be relatively light, take up little space, and use little power. The size and weight of the imager is especially important during deployment where added satellite weight may result in a more expensive and/or difficult launch. Present scanning imagers are relatively large, bulky, complex, and expensive. The present scanning imagers require relatively complex mechanical devices for scanning.
Most present remote sensing satellites utilize scanning imagers. The scanning imagers mechanically image one pixel at a time across a first row. As soon as the transmission is completed across the first row, the process is repeated in a second row, and so on until each row in the image is transmitted. The entire process of scanning, transmitting, and receiving a complete image can take more than twenty minutes. Since each detector spends very little time scanning each location (pixel), it is difficult to provide radiometrically accurate information at each pixel location. The equipment associated with these scanning systems is also relatively expensive. The problem with creating an image over a period as lengthy as tens of minutes is that the field of view of an imager at the beginning of the period is often different from the objects in its field of view even a few minutes later. For example, low earth orbit satellites travel a considerable distance over the Earth's surface during a one hour period, and it requires up to 12 hours for the same satellite to be located over the same geographic location to permit it to be re-imaged. It is therefore very difficult to co-register multiple images since co-registration requires multiple exposures of substantially the same field of view.
Since satellite imagers (also known as cameras) typically have many different functions, it is desirable that the imagers can be applied to sense different objects and conditions. Applying a single imager to one or more objects often requires a single imager to detect photonic radiation of differing wavelength ranges. It is often preferred that a single scene be imaged at different photonic radiation wavelength ranges to provide certain information relating to the object. Some of the desired wavelength ranges are in the visible photonic radiation range, while others may be in the infrared radiation range. Imagers which can detect a wide wavelength range of photonic radiation ranges often require the use of multiple imagers with distinct equipment or filters associated with each imager. It is difficult and expensive to provide a plurality of scanning imagers in a single satellite when each scanning imager takes up considerable space and weight.
The need for more timely imaging data, while providing similar quality images, is always desired. In some applications, it is desirable to be able to transmit complete images to the Earth within as little as one minute after the first pixel is imaged aboard the satellite. Current data imagers can not provide an accurate image within this time frame.
One difficulty with applying focal plane array imagers to satellites is lack of technique to precisely aim the focal plane array imager. In low earth orbit satellites, for example, the satellite travels at a high velocity with respect to fixed points on the Earth (or another planet) as the satellite orbits around the Earth (or another planet). This high rate of speed, and the associated jitter and drift, can result in blurred images even when they are exposed at high speeds. There must be some technique to precisely aim the staring focal plane array imager at some fixed location on the Earth's surface as the satellite orbits about the Earth, and keep the imager from recovering a distorted image. This aiming process requires adjusting the imager with respect to the satellite. Aiming of the staring focal plane array imager in satellites almost always occurs remotely from the satellite (or spacecraft) at some ground-based location. Continuous repeated observations of a single location on Earth by a small number of low earth orbit staring focal plane array imagers represents a considerable challenge.
A second difficulty with applying focal plane array imagers to the satellite application is that satellites produce a high frequency vibration known as jitter. Jitter is typically produced by variations of moving parts such as attitude control mechanisms or any other vibration producing mechanism located aboard a satellite. Jitter usually occurs at such a high frequency that attempts to create a highly magnified image using a staring or scanning imager mounted on a communications spacecraft result in a blurred image or subsequent comparison images in which the "same" earth location is displaced. Additionally, satellite focal plane array imagers drift with respect to a feature being imaged. Drift is a more gradual movement than vibration with the actual image Field of View (FOV) displaced from a desired image FOV. Jitter and drift, by themselves, have limited prior art attempts to apply staring focal plane array imagers to satellite applications.
A third problem with applying focal plane array imagers to spacecraft applications is that there must be some technique to control the temperature of the staring focal plane array imagers, especially when infrared radiation is being imaged to ensure proper operation of the imagers. Non-infrared staring focal plane array imager detectors do function well (exhibit a suitable signal to noise ratio) at the range of temperatures likely to be encountered in space. However, infrared staring focal plane array imager detectors must typically be cooled to below 100 degrees Kelvin to produce sensitive, accurate, and reliable imaging.
It would be desirable to be able to utilize imagers of relatively simple construction in satellite applications, which can also transfer images at a rapid rate. Prior art staring focal plane array imagers represent such a simple construction. Staring focal plane array imagers sense values for all the pixels of an entire image simultaneously over a very brief period, store the image in a data file, and transmit the data of the single image at whatever rate is desired. However, there are some difficulties with the characteristics of staring focal plane array imagers which limit these imagers from being used in satellite applications.
A desirable feature in making multicolored exposures, or for measuring a changing environment, is to be able to overlay time phased scenes. It is also desirable to be able to correlate successive images, which requires the imager to be very stable. Unfortunately, satellites which are partially dedicated to communication, and other tasks do not require the high stability required for sophisticated imaging, are often allowed to drift within a maximum allowable angular dead band in both attitude and location. Jitter and drift values that are acceptable for communication satellites are often outside the range permissible for scientific or observation imagery systems. To utilize available platforms, the stability problem must be overcome in order to be able to greatly improve such important applications of satellite images, which require rapid imaging, such as the calculation of dynamic motion fields (including wind and vapor fields), vorticity, and divergence for numerical forecast models, and short range forecasting.
From the above, it can be understood that even though focal plane array imagers have many characteristics which make their application to satellites desirable, it is also necessary to remedy some of the problems inherent in these imagers prior to their application to space environments. The present invention provides solutions to many of these above problems.